A typical nuclear reactor includes a core within which chain-reacting nuclear fuel material is located. The fuel material typically might be pellets of U.sup.235 or U.sup.238 or Pu.sup.239 which are encased in separate corrosion resistant heat conductive cans or cladding to form an elongated fuel element (also referred to as a fuel rod or fuel pin). A number of the fuel elements are grouped together and supported within a larger fuel assembly. The fuel assemblies are located then in a prearranged spaced matrix within the core of the reactor, with moderators or other form of control means being located in a different prearranged matrix within the core. The controlled presence of the fuel elements and control means regulate the extent of the nuclear reaction whereby neutron bombardment provides for thermal heating of the fuel elements and surrounding core structures. A reactor coolant is circulated through the core and fuel assemblies and over the fuel elements so as to cool them. The reactor coolant in turn is passed through a heat exchanger whereby a second coolant, commonly steam or water, is heated which second coolant is then expanded through appropriate steam expansion equipment for producing useful output typically for generating electricity.
Each fuel element, as noted, has a sealed exterior can or cladding, typically of stainless steel or zirconium alloy, so that the fuel material itself is sealed therein and is isolated from the coolant. This is needed firstly, to chemically isolate the nuclear fuel material from the coolant, and secondly to prevent the release of any radioactive fission products that may be generated in the nuclear reaction. Failure of the cladding, such as by localized melting or cracking, may thus release such fission products which would radioactively contaminate the circulating coolant which then would interfere with plant operation and maintenance. Further, a leaking fuel element could be the result of swelling that in turn further might block coolant flow and cause more extensive or costly overheating damage to the reactor. Thus, it is desirable to identify and locate a leaking fuel element as soon as possible so that the situation can be appraised and that fuel replacement procedures can be quickly handled with a minimal degree of cost and effort during subsequent reactor shutdown.
Most modern power reactors, particularly the breeder reactor where a liquid metal (sodium, for example) is used as the reactor coolant, have a sealed reactor system with an inert cover gas, typically argon that serves as a collector for any fission gases carried in the circulating coolant. To remove the fission gases, the cover gas must be removed from the reactor and processed in a cover gas cleanup system, such as in a bed of charcoal held at a cryogenic temperature, whereby the purified cover gas is then returned to the reactor. The fission gases commonly include the radioactive isotopes of xenon and krypton. This cleanup system can be operated continuously or only after a leaking fuel element has been detected.
A gamma ray radiation detector is commonly used to examine the cover gas for the presence of any of the gaseous fission products. However, this has little accuracy in identification specifics, so that further identification of the leaking fuel element and evaluation of the severity of the leak must still be made by other means.
Systems are being used to sample the cover gas and/or the coolant circulating in the reactor in an attempt to localize the leaking fuel element. The use of "sippers" has worked moderately well, whereby a portion of the coolant from selected fuel assemblies would be diverted to a remote sampling facility; and a multiple port valve would be shifted to periodically sample the coolant output from different proximate fuel assemblies. This system, however, does require prior assemblied clusters of coolant lines and valves, so it would not be practical in most existing power reactors not having the required hardware.
The concept of gas tagging is also known, being taught, for example, in the U.S. Pat. No. 3,632,470 assigned to General Electric Company; and U.S. Pat. Nos. 3,663,363 and 3,746,614 assigned to the U.S. Government. In gas tagging, stable isotopes of a gas are isolated in proportioned percentages of concentration to one another as a means for establishing unique combinations of such isotopes. The unique combinations of such isotopes, along with a filler gas perhaps of helium, would then be sealed within the different fuel elements as they were manufactured. The filler gas might comprise perhaps 90% of the gas mixture and would provide effective heat transfer between the fuel material and the fuel element cladding. The different fuel elements with their unique tags would be cataloged according to some matrix in the reactor core. Upon a breach of fuel element cladding, the unique "tag gas" mixture would escape to the coolant and would ultimately be carried to the cover gas area. Mass spectrometric analysis of the cover gas would give the weighted presence of the isotopes, and therefore identify the unique "tag gas". The corresponding fuel assembly "leaker" might then be identified according to the matrix catalog.
U.S. Pat. No. 3,632,470, proposes using the three stable non-radioactive isotopes of neon: Ne.sup.20, Ne.sup.21 and Ne.sup.22. However, the teaching has been inoperative in practice because the mass of the cover gas atoms (which typically is argon) to that of the neon tag gas atoms, is almost two to one. Thus, a doubly ionized argon isotope (Ar.sup.40++) looks almost identical on a mass spectrometer to a singly ionized neon isotope (Ne.sup.20+), and a precise analysis is difficult if not impossible to make.
U.S. Pat. No. 3,663,363 proposed using the xenon 124-130 isotopes as the tags. Although these isotopes would not normally be among the fission products generated in the reaction, the xenon 131-136 isotopes and the krypton 83-86 isotopes are the most common gaseous fission products. Consequently, the tag detecting system must attempt to isolate tag and fission isotopes from the same xenon family, which in effect greatly dilutes the concentration of the tag isotopes. Where the cover gas cleanup system is operated continuously, this in effect means that is is competing with the tag gas recovery and detecting systems for the same xenon isotopes. When the cover gas cleanup system can be stopped and only the tag gas recovery system operated, the xenon tags are diluted by the background blanket of fission product xenon and it becomes increasingly difficult to separate the tags from the fission gas. Notwithstanding these shortcomings, tag detecting systems of this type have been used in several liquid metal fast breeder reactors with varying degrees of success for many years.
While helium and argon have been proposed for possible use as the cover gas in the liquid metal fast breeder reactor, in practice argon has been used almost 100% of the time as the cover gas because of the relative ease of containment. The only gases suitable as failure "tags" in fuel assemblies are the noble gases of xenon, krypton, argon and neon. The argon cover has precluded the use of argon tags because of course, the tag isotopes could not be detected against the huge background of natural argon. As noted, neon tags cannot be used because the presence of doubly-ionized argon in the mass spectrometer interferes with the ability to resolve the neon tags. For these reasons, most tagging systems propose using either isotopes of xenon, or mixtures of isotopes of xenon and krypton. The high cost and complexity of xenon or xenon/krypton tags discourage their use in large-scale reactors where 600-800 unique tags would be required.
Furthermore, xenon is very difficult to extract from air, and by far the most difficult to enrich isotopically by thermal diffusion columns. This is so since as the atomic mass increases, the fractional difference in mass between adjacent isotopes becomes smaller. While Xe.sup.128 can be produced in essentially pure form by transmutation of iodine in a thermal nuclear reactor (albeit at a large expense), there exist only a finite number of enrichments for the remaining stable isotopes. Thus, the maximum mole percent attainable in commercially available enrichments is 40% for Xe.sup.124, 16% for Xe.sup.126, and 70% for Xe.sup.129. Moreover, for any given enrichment of one of these isotopes, the relative ratios of the remaining isotopes are essentially fixed.
These considerations impose severe physical constraints on the range of unique tag compositions that can be obtained with a xenon or xenon-krypton system of tags.
Another and most significant drawback on the xenon/krypton tag system is that when a fuel element has ruptured and fission gases have been detected the cover-gas cleanup system must run continuously to strip out the radioactive xenon and krypton fission gases. This means that any released tags of xenon or krypton will be subject to this cleanup system and likely disappear completely within a short time, viz., less than a few hours. Thus, if a tag is missed for some reason during this brief time, it is likely that the leaker which released that tag will not be identified.
Yet another drawback with xenon and xenon/krypton tags is that the principal tag ratios change substantially upon irradiation in the reactor. The most common two ways of dealing with these large changes, both of which increase the cost of tagging, are: (1) to track the compositions of every tag in the reactor by analytical and empirical means as a function of irradiation history; and (2) to provide sufficient spacing between adjacent tag ratios so that no tag can "burn into" a neighboring tag during irradiation. This required wide spacing of the xenon/krypton tags has the added drawback of using much more of the most expensive isotopes.
U.S. Pat. No. 3,746,614 use the three stable isotopes of Au.sup.197, Sb.sup.121, and Pt.sup.198 in slightly different weight ratios to one another as part of the bond coating over sodium bonded fuel elements. Thus the unique tag ratios of the different fuel elements can be identified by gamma spectrometric assay, and a fuel element tagging catalog can be used to pinpoint the element location in the reactor core. However, the coolant must leak through a failed cladding and contact the fuel element coating before the resulting tag would be released to flow with the coolant past the detecting area; and as the tag is a solid, the system is almost completely insensitive to a gas leaker.